Apparatus and method for thermally stripping volatile organic compounds from soil using a recirculating combustible gas

ABSTRACT

A transportable thermal stripping system for removing volatile organic compounds (VOC) from soil is provided. The system includes a vessel in which contaminated soil is placed into contact with a hot gas. The gas is discharged from the vessel and ambient air is introduced into the gas to ensure that the gas remains combustible. A blower circulates the gas to a cyclone separator where fine soil particles and ash are removed from the gas. Next the VOC in the gas are removed by combustion in a burner fired with a supplemental fuel. A portion of the combusted gas is exhausted to atmosphere and the remainder is recirculated to the vessel where the process is repeated.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application U.S.Ser. No. 537,089, filed Jun. 12, 1990, now U.S. Pat. No. 5,052,858,issued Oct. 1, 1991, the disclosures of which are hereby incorporated byreference in their entirety.

FIELD OF INVENTION

The present invention is directed to an apparatus and method forremoving volatile organic compounds ("VOC"), such as petroleumhydrocarbons, from soil using a hot gas for stripping the VOC from thesoil.

More specifically, the current invention is directed to such anapparatus and method in which the stripping gas is combusted after ithas volatilized the VOC, whereupon a first portion of the hot strippinggas is recirculated to minimize energy consumption and a second portionof the hot stripping gas is exhausted to atmosphere after cleanup.

BACKGROUND OF THE INVENTION

The present invention is directed to an apparatus and method forremoving volatile organic compounds, such as petroleum hydrocarbons,from soil, as a means for the environmental clean up of a dump site,landfill or spill site.

Environmental laws have imposed requirements that remedial measures betaken in dump sites, landfills and following chemical spills, leakagesor other accidents. This is particularly important in order to preventchemicals from contaminating ground water due to percolation through thesoil. When the chemical contaminant is a volatile organic compound, theremedial measures may involve removing such compounds from the soil byvolatilization. Such volatilization can readily be accomplished byheating the soil in a furnace. However, this method requires that thesoil be transported to a suitable processing facility for treatment. Forall but very small clean ups, this method is prohibitively expensive inview of the high transportation costs required.

Consequently, it would be desirable to develop an apparatus and methodfor removing volatile organic compounds from contaminated soil whileon-site, using as a means for such removal, equipment that is easilytransported to that site. Since, with portable equipment, the volume ofsoil which can be processed at any one time will necessarily be limited,it would be desirable to provide a means for accelerating thevolatilization rate so that the total clean up time is minimized.

Applicant's U.S. Pat. No. 5,052,858 disclosed an apparatus and methodfor removing volatile organic compounds from contaminated soil whileon-site, using equipment that is easily transported to the site.According to that scheme, all of the hot gas used to strip the VOC fromthe soil was exhausted directly to atmosphere without cleanup. Althoughthis approach affords an excellent response to environmental needs inmany situations, it suffers from two drawbacks. First, the gas exhaustedto atmosphere contains substantial heat which was added to the gas byburning a relatively expensive fuel--specifically, propane. Hence, thethermal efficiency and economy of the thermal stripping process isimpaired. Second, since the stripping gas is laden with VOC, which are asource of air pollution, the amount of soil which can be treated per dayis often limited by local environmental regulations.

Consequently, it would be desirable to provide a portable thermalstripping apparatus which minimizes the amount of supplemental fuelwhich must be burned to heat the stripping gas and which removes atleast a portion of the VOC in the gas prior to exhausting it toatmosphere

SUMMARY OF THE INVENTION

It is an object of the current invention to provide a portable apparatusand a method for removing VOC from soil utilizing a hot stripping gas inwhich the amount of supplemental fuel which must be burned to adequatelyheat the stripping gas is minimized and at least a portion of the VOC inthe gas are removed prior to exhausting the gas to atmosphere.

These objects are accomplished in a method for removing VOC from soil by(i) loading the soil into a vessel, (ii) introducing a hot gas into thevessel and causing the hot gas to come into contact with the soil,thereby partially cooling the hot gas and volatilizing the volatileorganic compounds into the gas, (iii) discharging the gas from thevessel, (iv) introducing air into the discharged gas, thereby forming acombustible gas, (v) burning the combustible gas, thereby oxidizing atleast a portion of the volatile organic compounds volatilized into thegas and reheating the gas, (vi) discharging a first portion of thereheated gas to atmosphere, (vii) recirculating a second portion of thereheated gas to the vessel and causing the second portion of the gas tocome into contact with the soil, thereby partially cooling the hot gasand volatilizing the volatile organic compounds into the gas, and (viii)repeating steps (iii)-(vi).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one embodiment of the thermalstripper system according to the current invention.

FIG. 2 shows the rear end of the drum shown in FIG. 1, without thedischarge chute installed.

FIG. 3 is a cross section of the drum taken through line III--III shownin FIG. 2, illustrating the auger mechanism.

FIG. 4 is a perspective view of the air heater shown in FIG. 1.

FIG. 5 is a schematic diagram of an alternative embodiment of thethermal stripping system according to the current invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

There is shown in FIG. 1 a schematic diagram of one embodiment of aportable thermal stripping system according to the current invention.The primary components of the system are a cement truck 1, an air heater2;, and a fuel supply 20. The cement truck is comprised of a tractor 22,connected to a truck bed 23 on which is supported a drum 2. The drum iscapable of rotation about its longitudinal axis in both the clockwiseand counter clockwise directions at varying speeds through the action ofa hydraulic drive mechanism 38 which is disposed in the truck bed andpowered by the truck engine. In the preferred embodiment, the drivemechanism 38 is capable of rotating the drum at speeds up to 50 RPM. Thedrum rotational speed may be controlled by varying the engine RPM byadjustment of the throttle setting. At any given throttle setting, thedrum rotational speed may also be adjusted by operating a valve in thehydraulic drive system. The initiation and direction of drum rotation iscontrolled through panel 39, which features start, stop andforward/reverse rotation switches. The drive system described above isof the standard type used in cement trucks.

The drum is an approximately cylindrically shaped vessel. In thepreferred embodiment, the drum is of the standard cement truck type,such as Model Rex 770, which may be purchased from Rex Works, Inc.,Milwaukee, Wis., 53201. Preferably, the drum has an axial length ofapproximately 15 ft., a maximum diameter of approximately 8 ft., and auseful capacity in the range of 8-10 cubic yards of soil, suchdimensions making the drum suitable for ready transportation usingconventional cement truck tractors. However, the principles disclosedherein are equally applicable to larger or smaller drums. The drumfeatures an opening 24 in its aft end, to which is attached an inlethopper 3 and a discharge chute 4. As explained further below, soil to beprocessed enters and exits the drum through the inlet hopper anddischarge chute, respectively.

As shown in FIG. 1, in the preferred embodiment, the axis of the drum isinclined with respect to the horizontal at an angle in the range ofabout 10-15 degrees. Note that the aft end, in which the opening 24 isdisposed, is higher than the front end.

An auger mechanism is disposed within the drum. The auger mechanismperforms two important functions grinding/churning of the soil andtransporting of the soil into and out of the drum. Since volatilizationoccurs through the surface of the soil particles, removal ofcontaminants from large clumps of soil would require prohibitively longprocessing times. The grinding action of the auger mechanism minimizesparticle size, thereby increasing the surface area of the soil. Thus,pretreatment of the soil, even heavy clay-like soil, is not required.The churning action of the auger mechanism constantly exposes thesurfaces of the particles to fresh quantities of hot air, supplied tothe drum as explained below, thereby mechanically aerating the soil.Thus, the auger mechanism minimizes the time required for adequatevolatilization.

As shown in FIG. 3, in the preferred embodiment, the auger mechanism iscomprised of two continuous helical baffles 25, 26 which extend thelength of the drum and terminate at the opening 24. Each helix isaffixed at its periphery to the inner surface of the drum, for example,by welding, and projects radially inward into the drum. The axis of eachhelix is coincident with the longitudinal axis of the drum. Both helixesare of similar configuration--that is, both are right hand or both areleft hand helixes. As shown in FIG. 3, the helixes are axially offsetfrom each other by one-half pitch. The radial height of each helicalbaffle varies as the drum diameter varies along the length of the drum.For a drum with a maximum diameter of approximately 8 feet, the maximumradial height of the helical baffles is approximately 12-18 inches inthe preferred embodiment.

Although flat baffles are shown in FIG. 3, more complex shapes, such asbaffles whose inboard edges are bent at 45° or 90° to the body of thebaffle, may be used to increase the churning capacity of the augermechanism. Moreover, although two baffles are used in the preferredembodiment, as will be obvious to those skilled in the art, the augermechanism may be comprised of a single helical baffle or three or morehelical baffles similarly affixed to the drum.

The grinding/churning actions of the auger mechanism are achieved byrotating the drum, which in turn results in rotation of the auger aboutits axis using the aforementioned hydraulic drive mechanism. Thisarrangement is standard for cement trucks. Alternatively, the augermechanism could be comprised of one or more helical baffles affixed attheir inner edge to a shaft, the center line of the shaft beingcoincident with the longitudinal axis of the drum. In this embodiment,the drum would remain stationary and the grinding/churning action of theauger mechanism would be achieved by rotating the shaft within the drum.

As previously discussed, the auger mechanism serves to transport, aswell as grind and churn the soil. Thus, rotation of the drum in onedirection serves to carry the soil up the incline from the bottom of thedrum and pushes it out onto the discharge chute. In this respect, theauger mechanism functions as a screw-type conveyor. Rotation of the drumin the opposite direction results in the aforementioned grinding andchurning action. The direction of rotation, clockwise or counterclockwise, which causes the transporting and grinding/churning actions,respectively, depends on whether the auger helix is right or lefthanded, and may be arbitrarily chosen. As will be apparent to thoseskilled in the field of screw-type conveyors, the minimum pitchsufficient to efficiently carry soil out of the drum is related to theincline angle of the drum--that is, the greater the incline, the smallerthe helical pitch required.

Returning to FIG. 1, it can be seen that the air heater 21 is comprisedof a cylindrical vessel 5, a fan 6, a duct 11, a skid 14, a burner 7 anda control module 10. The burner 7, which is of the in-stream type(meaning the air to be heated flows over the burner), is disposed withinthe vessel 5 as shown in FIG. 4, and burns a liquid or gaseous fuel. Theburner is supplied with fuel from the fuel tank 12, which is mounted onskid 34. Piping 8 connects the fuel tank to the burner and features adetachable coupling 37 at the piping/burner interface. In the preferredembodiment, the burner operates on propane fuel and has a maximum heatrelease capacity of 2.5 mbtu/hr.

As shown in FIG. 4, a fan 6 is connected to the aft end of the vessel 5.Ambient air 29 is drawn into the vessel by the fan and forced to flowover the burner 7, providing it with the necessary combustion air. Inthe preferred embodiment, the fan is of the centrifugal type andfeatures backward inclined blades and has a maximum capacity of 4000SCFM. Other types of fans, including axial fans, could also be used,provided they can deliver the flow of air required for goodvolatilization, which in turn depends on the size selected for the drum.In the preferred embodiment, the fan features an indirect drive throughbelt 30 from motor 40. The output of the fan is set by varying the speedof the fan by loosening or tightening the belt 30. Alternatively, theoutput of the fan could be varied by the use of variable inlet guidevanes, variable inlet or outlet dampers, or a variable speed motor.

After being heated by the burner 7, the air 15 is discharged from thevessel 5 through a nozzle 31 at the front end of the vessel. Note thatthe walls of the vessel approaching the nozzle are gradually tapered toinsure smooth flow and minimize the backpressure in the system. Afterexiting the nozzle, the hot air 15 flows through the duct 11, as shownin FIG. 1. A thermocouple 13 is disposed at the inlet to the duct andsenses the temperature of the hot air. Since the vapor pressure ofvolatile organic compounds increases with increasing temperature, thehotter the air temperature, the more rapid the volatilization rate.However, the temperature should not be so high as to cause incinerationof the soil. Therefore, in the preferred embodiment, the temperature ofthe hot air 15 discharging from the vessel is maintained in the800°-1500° F. range, depending on the type of contaminant. Moreover, asa result of the intervening duct 11, the flames generated by the burnerdo not penetrate into the drum, further ensuring volatilization withoutincineration.

The temperature of the hot air is maintained in the desired range by thecontrol module 10, which may be a micro-computer. The desired airtemperature is manually entered into the control module. In addition,the output signal from the thermocouple 13 is transmitted to the controlmodule through the conductor 32. Using means well known to those skilledin the art of flow control systems, the control module compares themeasured temperature to the desired temperature and transmits an outputsignal to flow control valve 9, through conductor 33, the amplitude ofwhich is proportional to the deviation in the measured temperature fromthe desired temperature. Valve 9 is disposed in the piping 8 between thefuel tank 12 and the burner 7, and regulates the fuel flow to the burnerin response to the amplitude of the signal it receives. Thus, the fuelflow to the burner 7, and hence the temperature of the discharging air15, is increased or decreased by operation of the control module on theflow control valve, as required to maintain the correct temperature. Inaddition to the flow control valve, other piping, valves and controlfeatures are necessary to reliably start and operate the burner and toensure compliance with local safety regulations. However, suchrequirements are well known to those skilled in the art of burnersystems and are not described in detail herein. In addition toregulating fuel flow, the control module performs the sequencingfunctions necessary to start and operate the fan and burner.

As shown in FIG. 1, the duct 11 terminates at the inlet hopper 3 of thedrum. As shown in FIG. 3, the outside diameter of the duct 11 is onlyslightly smaller than the inside diameter of the hopper throat 19. Thus,no connection is required to mate the duct to the drum other thaninserting the duct into the hopper throat.

The hopper is aligned so that its discharge 28 is concentric with thedrum opening 24, thereby directing the incoming hot air 16 into the drumalong its axis. The hot air 17 circulates within the drum and, as aresult of the churning action of the auger mechanism, comes into directcontact with the soil particles. Thus, the combined effect of the augermechanism and the air heater results in vigorous mechanical aeration andheating of the soil. This aeration and heating promotes rapidvolatilization of the volatile organic compounds.

As explained above, the hopper discharge 28 is centrally disposed in thedrum opening. The annular portion 27 of the drum opening 24 whichsurrounds the hopper discharge, shown best in FIG. 2, serves as theexhaust port for the drum. In the embodiment shown in FIG. 1, aftercirculating through the drum, the hot air 18 is exhausted directly intothe atmosphere through the annular portion 27. The exhaust air 18carries with it the vapors released as a result of the volatilization.Since the volatilization rate of the volatile organic compoundsdecreases with increasing pressure, the flow area of the annular portion27 should be sized to prevent excessive build up of pressure in thedrum. In the preferred embodiment, the diameter of the hopper discharge28 is only about one third that of the drum opening 24 to ensure ampleexhaust flow area.

As a result of the hot air being introduced at the center of the drumopening and the exhaust port being the annular portion surrounding thiscenter portion, the hot air flows in the circulating flow path shown inFIG. 3. Entering air flows along the core of the drum, reversesdirection at the end wall 39 of the drum and then flows out of the drumalong the annulus surrounding the drum core. Not only does this flowpath provide ample opportunity for contact between the soil particlesbeing churned by the auger mechanism, it also reduces the tendency for"dusting"--that is, there is less of a tendency for fine soil particlesto be entrained in air 18 exhausting from the drum if a circulating, asopposed to a straight through, air flow path is utilized. In addition,the helical baffles 25, 26 act as separators, further preventing finesoil particles from being carried out of the drum.

According to the embodiment of the invention shown in FIG. 1, nopost-processing clean up of the air exhausting from the drum isemployed. Thus, the reduction in dusting afforded by the drum design isan important factor in obtaining the simplicity of this embodiment ofthe thermal stripping system. It should be noted that the permissibilityof directly discharging volatile organic compounds into the atmospheremay be prescribed by federal and local environmental regulations,depending on the type of contaminant in the soil. However, discharge ofmany volatile organic compounds, such as those associated with gasolinespills, is often permitted subject to certain limitations--usually amaximum rate, expressed as pounds per day of contaminant. Thus, by (i)confining the use of the thermal stripper system to volatile organiccompounds which can be safely exhausted directly into the atmosphere inlimited quantities, (ii) minimizing the tendency for soil particles tobe entrained in the exhaust air, and (iii) limiting the amount of soilprocessed per day, the cost and complexity of an exhaust gas clean upsystem can be avoided. As discussed further below, according to a secondembodiment of the current invention, an exhaust gas clean up system isprovided when these limitations on the operation of the thermalstripping system are unacceptable.

An important feature of the thermal stripping system is its readytransportability to the clean up site. The aeration device in the formof a cement truck 1 is easily relocated. Wheels 35 and hitches 36 areattached to the air heater skid 14 and the fuel supply skid 34, makingthese components entirely portable also. Moreover, upon arrival at theclean up site, one need only to connect the fuel tank to the burnerpiping at coupling 37, and the insert the hot air duct 11 into thehopper 3, to place the system into operation.

Once at the clean up site, to carry out the method of the invention, thesoil to be treated is loaded into the drum through the inlet hopper 3using standard excavation equipment, such as a front loader. The drum isrotated during the loading process so that the auger mechanism draws thesoil from the hopper into the drum. This drum rotation is maintainedduring the entire soil processing period. To ensure that adequate roomexists for the circulation of the hot air, the drum is only partlyfilled with soil.

After the soil is loaded into the drum, the air heater 21 is moved intoposition adjacent the drum and the duct 11 is connected to the hopper.The fuel skid 34 is also moved into position and the piping 8 isconnected to the burner 7. The fan 6 is then started, followed by theburner 7. As previously mentioned, the control module 10 sequences thestart up of the fan and burner and regulates the fuel flow control valve9 so as to maintain the hot air 15 discharging from the air heater atthe appropriate temperature. The air temperature is preselected based ona pre-treatment soil analysis, as discussed below. The hot air thencirculates through the rotating drum as previously discussed. As aresult of the drum rotation, the auger mechanism grinds and churns thesoil, resulting in vigorous aeration of the soil with the hot air. Thisheating and aeration volatilizes the volatile organic compounds presentin the soil. The volatilized organic compounds are then preferablyexpelled into the atmosphere.

During soil processing, the drum rotational speed must be maintained atthe appropriate level for efficient grinding/churning action. Arotational speed which is too slow will result in most of the soil lyingstagnate in the lower portion of the drum. A rotational speed which istoo high will result in a centrifuge action, causing the soil to clingto the periphery of the drum. The appropriate speed for maximumgrinding/churning efficiency will depend on the geometry of the augermechanism, the diameter of the drum and the density and cohesiveness ofthe soil. In the preferred embodiment, the rotational speed of the drumis maintained within a range of about 4-18 RPM.

The processing described above is continued until the concentration ofthe volatile organic compounds in the soil is reduced to an acceptablelevel. As those skilled in the art will recognize, the acceptable leveldepends on the type of organic contaminant, and may also vary with localenvironmental regulations, but is generally in the range of 10 PPM byweight for the volatile organic compounds associated with gasolinespills.

The length of the processing time required to reduce the concentrationof volatile organic compounds to an acceptable level will depend on avariety of design, operating, and soil parameters, as will be readilyapparent to those skilled in the art. As previously discussed, thedesign parameters include the drum size, the geometry of the augermechanism and the drum incline angle. The operating parameters includethe drum rotational speed, the percent of the drum volume filled withsoil, the air flow rate, the temperature of the air entering the drumand the air pressure within the drum. Soil parameters include theparticle size distribution, cohesiveness, density, moisture content andtemperature of the soil, as well as the contaminant type (specifically,the vapor pressure of the volatile organic compound) and itsconcentration level.

For components of a given design, the operating parameters and therequired processing time period are initially determined based on apreliminary soil analysis. The required processing time is directlyproportional to the soil particle size, cohesiveness, density, moisturecontent and contaminant concentration level and indirectly proportionalto the soil temperature and the vapor pressure of the volatile organiccompound. Since the soil is treated in a batch-like fashion, there is nolimitation on the processing time. Moreover, the processing time can beset independent of the drum rotational speed. As those skilled in theart will recognize, clay-like soils will require longer processing time,whereas sandy soil will require shorter processing time. In general, ithas been found that a processing time between about 30-90 minutes isadequate for the clean up of most soils contaminated with petroleumproducts. In one experiment conducted by the inventors, 1900 cubic yardsof clay-like soil containing petroleum product contaminants wereprocessed over a 21 day period. Initial concentrations of petroleumhydrocarbons as high as 7000 PPM by weight were reduced in thisexperiment to less than about 10 PPM by weight by processing for about60 minutes at a drum rotation of 4 RPM and an airflow of 4000 CFM at1000° F.

Although the preliminary soil analysis provides initial operatingparameters and processing time estimates, samples of processed soilshould also be periodically analyzed to insure that the operatingparameters and processing time are adequate for the particularconditions of the clean up site. In the preferred embodiment, the soilis analyzed after each 50 cubic yards of soil has been processed.

After the required processing time has elapsed, the burner and fan areshut down and the duct is disconnected from the drum. The drum is thentransported to the soil storage area and its rotation is reversed, sothat the auger mechanism conveys the processed soil out of the drum andinto discharge chute 4. The drum is then reloaded with fresh soil andthe process repeated in a batch-like fashion until all of thecontaminated soil has been cleaned.

FIG. 5 shows a schematic diagram of an alternate embodiment 100 of thethermal stripping system according to the current invention. Thisembodiment features an exhaust gas clean up and recirculation system toincrease the thermal efficiency of the thermal stripper and to minimizethe amount of pollutant discharged to atmosphere.

The thermal stripper system 100 includes a cylindrical vessel 102 intowhich a predetermined quantity of contaminated soil is loaded. Thevessel 102 may be the rotating drum of a conventional cement truck, aspreviously discussed. However, other devices, such as rotary dryers,capable of bringing a gas into intimate contact with the soil may alsobe utilized. The other major components of the system are an aspirator105, a blower 107, a cyclone separator 109, a storage bin 119, a burner112, and a flow splitter 115. All of these components are placed intoflow communication with each other, and with the vessel 102 by ductwork123. Although for simplicity, only the portions of the ductworkimmediately adjacent the vessel 102 are shown in FIG. 2, it should beunderstood that the ductwork 123 connects each of the components shownby directing the flow of hot gas from component to component asindicated by the arrows 103, 106, 108, 111, 114 and 117 designating thehot gas flow path.

As explained further below, a hot recirculating stripping gas 117,heated into approximately the 800°-1500° F. temperature range, isdirected to the vessel 102 by duct 123 and introduced therein by aninlet port 122 formed in a cover 120 sealing the opening in the end ofthe vessel. As previously discussed, in the vessel 102, the hot gas 117comes into intimate contact with the contaminated soil causing the VOCcontained therein to be volatilized into the gas. Fine particles of soilare also entrained in the gas. The gas 103, now cooled to approximately500° F., is discharged from the vessel 102 via exhaust port 121 formedin cover 120.

Air is then introduced into the discharged gas 103 by directing the gas,via duct 123, to the aspirator 105. In the preferred embodiment, theaspirator 105 is a funnel-like device having a perimeter which is opento the atmosphere. The gas 103 is directed to the center of theaspirator 105 while ambient air 104 is drawn into the perimeter of theaspirator by the blower 107. Air 103 is introduced int the gas 103 toensure that sufficient oxygen exists for combustion of the gas, asexplained further below. Although an aspirator is used in the preferredembodiment, those skilled in the art will appreciate that other means ofintroducing ambient air into the gas may be utilized, such as anadditional blower to inject the air into the gas.

The oxygenated and further cooled gas 106 is then directed by the duct123 to the blower 107, which may be of the centrifugal type. The blower107 pressurizes the gas and directs it, via the duct 123, to a cycloneseparator 109. As is conventional, in the cyclone separator 109, the gasis carried downward in a vortex created by the internal surface of thecyclone separator. Solid particles--in this case, fine soil particlesentrained in the gas when it circulated through the vessel 102 and ashfrom combustion of VOC volatilized in the burner 112--are centrifugedtoward the outer edges of the vortex and travel downward to a solidsdischarge port where the particles 110 exit the separator and arecollected in a storage bin 119. The cleaned gas 111 then flows upwardthrough the cyclone separator 109 end exists through a gas dischargeport, whereupon it is directed by duct 123 to a burner 112.

The burner 112, which may be of in-stream type previously discussed, issupplied with supplemental fuel 113 from a fuel supply (not shown) viaconduit 124. In the preferred embodiment, the fuel is propane suppliedfrom a tank which is included in the transportable apparatus, aspreviously discussed. The flow of fuel 113 to the burner 112 isregulated, using techniques well known in the art, by the controlmodule, as previously discussed. The burner control system receives asignal from a temperature sensor 118 disposed in duct 123 downstream ofthe burner and regulates the flow of fuel 113 so as to maintain thedesired temperature of the gas 114 exiting the burner. In the preferredembodiment, the gas 114 is heated such that its average temperature isin the approximately 800°-1500° F. range, most preferably approximately1400° F., thereby ensuring good volatilization in the vessel 102 whenthe gas is recirculated thereto, as explained below.

In the burner 112, the flame temperature is sufficient to causeoxidation of a substantial portion of the VOC previously volatilizedinto the gas, thereby transforming the VOC into less harmful substances,such as carbon dioxide and water. A portion 116 of the gas 114 from theburner 112 is exhausted to atmosphere through a flow splitter 115. Inthe preferred embodiment, the gas 116 exhausted to atmosphere representsapproximately one half of the flow of gas 114 from the burner 112. As aresult, the amount of air 4 introduced into the gas 103 by the aspirator105 is approximately equal to the flow of gas 103 discharging from thevessel 102. Thus, the flow of gas into the drum remains constant andsufficient oxygen is added to ensure that the gas supplied to the burner112 is combustible. It is important to note that the removal ofparticulate matter in the cyclone separator 109 and the removal of VOCin the burner 112 allows considerably more soil to be treated withoutexceeding local environmental limits for these substances.

The flows splitter 115 recirculates a second portion 117 of the gas 114from the burner 112 to the vessel 102 via duct 123, whereupon it entersvia inlet port 122 and the cycle is repeated.

As can be readily seen, since one half of the gas discharged from thevessel 102 is recirculated thereto by the duct 123, the sensible heatcontained therein is not lost. Thus, the amount of fuel 113 which mustbe consumed by the burner 112 to achieve the proper temperature of thegas 117 entering the vessel is considerably reduced. It should also beappreciated that the combustion of the VOC in the gas 111 not onlyremoves pollutants, it contributes to the heat release in the burner112, thereby further reducing fuel consumption.

Thus, as a result of the exhaust gas cleanup and recirculation systempreviously described, both the maximum capacity and the energyefficiency of the thermal stripper system are considerably increased.

According to an important aspect of the current invention, the entirethermal stripping system, including all the components of the exhaustgas cleanup and recirculation system shown in FIG. 5, are readilytransportable by mounting onto mobile skids, as previously discussed.

Although the thermal stripper system has been described by reference tospecific components, as will be clear to those of skill in the art,other components are also suitable for use in the system--for example,the centrifugal blower 107 could be replaced by other types of fluidmoving apparatus, such as an axial fan, the cyclone separator could bereplaced by other particle removal devices, such as bag houses, and aburner other than a gas fired in-stream type could be utilized.Accordingly, the present invention may be embodied in other specificforms without departing from the spirit or essential attributes thereofand, accordingly, reference should be made to the appended claims,rather than to the foregoing specification, as indicating the scope ofthe invention.

What is claimed is:
 1. A method for removing volatile organic compoundsfrom soil, comprising the steps of:a) loading said soil into an openingin a drum; b) transporting said soil into said drum by rotating saiddrum in a first direction; c) flowing a hot gas through said drum; d)mechanically causing said hot gas to come into contact with said soil,thereby volatilizing said volatile organic compounds into said hot gas;e) exhausting said hot gas after said hot gas has flowed through saiddrum; f) introducing air into said exhausted gas; g) combusting saidexhausted gas after said introduction of said air therein; h)recirculating at least a portion of said combusted gas to said drum; i)repeating steps (c)-(h); and j) discharging said soil from said drumthrough said opening by rotating said drum in a second direction.
 2. Themethod according to claim 1, wherein the step of introducing air intosaid exhausted gas comprises aspirating said air into said exhaustedgas.
 3. The method according to claim 1, wherein the step of combustingsaid exhausted gas comprises introducing supplemental fuel into saidexhausted gas and combusting said volatile organic compounds and saidsupplemental fuel.
 4. The method according to claim 3, wherein saidvolatile organic compounds are removed from said soil in a batchprocess, and wherein:a) the step of loading said soil comprises loadinga batch of said soil into said drum; b) the step of flowing a hot gasthrough said drum comprises flowing said hot gas into said drum at atemperature within a predetermined range; and c) the step of introducingsaid supplemental fuel into said exhausted gas comprises controlling thequantity of said supplemental fuel introduced so as to maintain said gasin said predetermined temperature range.
 5. An apparatus for removingvolatile organic compounds from soil, comprising:a) a drum for holding apredetermined quantity of said soil; b) gas transporting means, in flowcommunication with said drum, for directing a hot gas into said drum andfor discharging said hot gas from said drum and for recirculating saiddischarged gas back into said drum; c) an auger mechanism disposed insaid drum, said auger mechanism having means for grinding and churningsaid soil when said auger mechanism is rotated in a first direction,thereby volatilizing said volatile organic compounds and entrainingparticles of said soil into said hot gas directed into said drum, andmeans for transporting said soil out of said drum when said augermechanism is rotated in a second direction; and d) means, in flowcommunication with said gas transporting means, for removing particlesfrom said gas discharging from said drum.
 6. The apparatus according toclaim 5, further comprising means, in flow communication with said gastransporting means, for burning said volatile organic compoundsvolatilized into said hot gas.
 7. The apparatus according to claim 6,wherein said means for burning said volatile organic compoundsvolatilized into said hot gas comprises means for introducingsupplemental fuel into said hot gas.
 8. The apparatus according to claim7, further comprising means for controlling the quantity of saidsupplemental fuel introduced into said gas so as to maintain said gas ina predetermined temperature range.